Truncation is
like a wildcard. Added to the stem of a word, it will find that
stem plus anything that comes after it. The symbol used to truncate
a word depends upon the index, database, or Web search engine you
are using.

psychol*

will
return records on psychology, psychological, psychologist.* is used as the truncation symbol, for example, in EBSCO and ProQuest databases.

environ!

will
return records on environment, environments, environmental. ! is used as the truncation symbol in LexisNexis databases.